The "walking on the wall" method is always a good place to start.
That said if you can't do it (either due to too much body fat/muscle in the way or not a long enough penis), you can't do it. Don't try to injure yourself doing it (it hurts enough being able to do it!).
Also I find that unlike muscle work-outs, you can do it every day basically, without causing too much soreness. I actually find if you leave too much time between "sessions", then back pain can increase...